Molly I'm sorry you had such an awful day yesterday. I mentioned the metaphor of the storm raging, downthread. Perhaps yesterday was its eye.
I've got to raise some suspicions I have, I'm afraid. How certain are you that he actually saw a psychologist yesterday? I remember thinking it was unusual that he managed to get an appointment on a Saturday at such short notice, but his responses on return, sound more like he's benefited from the twisted psychology of the OW, rather than a qualified, objective professional. Is it possible then, that he went to see the OW yesterday and didn't go near a psychologist?
I hope you're feeling stronger today and better able to get support from people who love you unconditionally, because my feeling is that the moment someone tries to blame you for his own behaviour, is the time for him to go. You now need support from people other than him and his parents - please reach out and get that help, whoever you decide it will be.
I think that when you felt that fear yesterday that something was going to unravel at the psychologist's and he was going to leave you, it was a very powerful subconscious instinct; almost as though you knew there was something not quite fitting about this "appointment" - and at the very least, his frame of mind approaching it.
I wonder whether one of your barriers to asking him to leave for a while is that you fear he will go to the OW, especially as she has a ready-made home for him? But much as I understand that, if he goes there, it will give you some answers going forward. If he doesn't go anywhere near her on the other hand - and starts to fight for you, then you will re-start this process on a much firmer footing and with greater hope that he wants to stay in this marriage, will take total responsibility for his behaviour and start looking inwards for blame, rather than opting for the cowardly refuge of deflecting it - and trying to pass it to you.